A pair of Clarkii's MIGHT be fine, but there is already another pair of clowns in the tank in question. Bringing the total to 4 clownfish. In addition to whatever else may/may not be in the tank.
And clownfish can get big, especially clarkii's. They would not thrive in a 30 gal/tank, in which there is probably 20 gal of water at best, depending on substrate, LR, etc.
Once again, my humble .02
Hermit